Field of the Invention
The present invention relates to a treatment liquid for a trivalent chromium conversion coating and a treatment method of a metal substrate.
Description of the Related Art
In general, zinc or zinc alloy plating is widely used as a method for rust prevention of iron materials and iron parts. Zinc is, however, a metal to rust easily, so that the direct use thereof immediately causes the occurrence of white rust, which is the rust of zinc. Accordingly, formation of a further protective coating is commonly required.
In the field of conversion coating treatment to form a protective layer, hexavalent chromating used to be frequently employed. Due to environmental problems, however, trivalent chromium conversion coating is mainly used at the present time. The art related to the field is described in prior literature such as Japanese Patent Laid-Open No. 2000-509434, Japanese Patent Laid-Open No. 2005-240068.
A treatment liquid for a trivalent chromium conversion coating for zinc or zinc alloy plating typically contains a cobalt compound in order to improve the corrosion resistance. Japanese Patent Laid-Open No. 2000-509434, Japanese Patent Laid-Open No. 2003-166074, Japanese Patent Laid-Open No. 2007-321234, Japanese Patent Laid-Open No. 2005-240068.
However, the influence of cobalt compounds on environment has been indicated. A part of cobalt compounds are already registered on the SVHC (Substance of Very High Concern) list of REACH (Registration, Evaluation, Authorization and Restriction of Chemicals) regulations. Other cobalt-containing compounds are also registered in the same way. In Japan, measures for use of cobalt have been enhanced by Industrial Safety and Health Law Enforcement Ordinance and Ordinance on Prevention of Hazards due to Specified Chemical Substances. Accordingly, use of a cobalt compound contained in the treatment liquid for a black trivalent chromium conversion coating may be also restricted in the future.
A treatment liquid for a trivalent chromium conversion coating with addition of a titanium compound instead of a cobalt compound is described in Japanese Patent Laid-Open No. 2014-159627. The titanium compound-containing treatment liquid for a trivalent chromium conversion coating described in the literature, however, has a problem that the stability of the treatment liquid, the corrosion resistance, and the appearance are hardly obtained in the conversion coating treatment in an actual production.
In view of the foregoing problem, it is an object of the present invention to provide: a treatment liquid for a trivalent chromium conversion coating, achieving excellent corrosion resistance and scratch resistance without containing a cobalt compound, with high stability; and a method for treating a metal substrate using the same.